fix(VCarouselItem): correctly renders placeholder slot in v-img

[Kapcash]

Description

Resolves #8452 from regression caused by the commit 2b975f5

The VCarouselItem component doesn't render the placeholder slot correctly.

It's failing because we're considering it's a scopedSlot instead of just a slot when the VCarouselItem renders the VImg. But on VImg there is a condition that only checks for the slots, not the scopedSlots:

// VImg.ts
__genPlaceholder (): VNode | void {
   // this.$slots.placeholder is undefined, while `this.$scopedSlots.placeholder` exists.
   if (this.$slots.placeholder) {
   }
}

So I just rendered the slot using a template children so the condition can pass and actually render the placeholder :)

I noticed this commit actually changed from slots to scopedSlot but I'm not sure what was the purpose of the change. Maybe I'm missing something that made it legit to use scopedSlot?

Note: do you want me to create an issue before, and link it on this PR?

Motivation and Context

How Has This Been Tested?

I added a unit test for this specific slot, testing it is correctly rendered within the VImg.

I had to update a snapshot for VOverflowBtn since it was failing. I guess it's failing on master too since I didn't touched this component.

Markup:

<template>
  <v-container>
    <v-carousel hide-delimiters>
      <v-carousel-item src="https://picsum.photos/510/300?random">
        <template #placeholder>
          I'm vanishing, haaaaa
        </template>
      </v-carousel-item>
      <v-carousel-item src="https://vuetifyjs.com/static/doc-images/cards/i-dont-exist.jpg">
        <template #placeholder>
          Bad image url -> show placeholder
        </template>
      </v-carousel-item>
    </v-carousel>
  </v-container>
</template>

<script>
  export default {
  }
</script>

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Improvement/refactoring (non-breaking change that doesn't add any features but makes things better)

Checklist:

• The PR title is no longer than 64 characters.
• The PR is submitted to the correct branch (master for bug fixes and documentation updates, dev for new features and backwards compatible changes and next for non-backwards compatible changes).
• My code follows the code style of this project.
• I've added relevant changes to the documentation (applies to new features and breaking changes in core library)